White cement

Many of us, it seems that cement is cement, it can be gray, perhaps with various additives, sometimes ash, slag or sometimes even something else. What stands out the white cement? What makes that kind is cement, but both its power characteristics and most importantly - white allow for many different and interesting applications?

Looking at the english wikipedia you can find out that in the production of cement in the makers trying to get rid of various chemical compounds that make the natural color of cement is shade of gray. In the production process of trying to get rid of chromium (green coloring), iron compounds (commonly used in chemistry as the base for the total weight of pigments cement), as well as manganese, copper, nickel or titanium. Cement endeavor, that the amount of chromium should not exceed 0.003%, manganese - 0.03%, and of iron - 0.35%. Other "pollution" are not a problem.

To make this process easier, the basic thing is to find deposits of limestone so pure as to contain the least undesirable additives. After addition of all other ingredients are thoroughly mixed and ground, then subjected to chemical processes at high temperature which, inter alia, take part calcium sulfate and calcium fluoride. The problem is to get the lowest possible content of iron, which quite strongly affect the deterioration of the quality of clinker, and thus also of cement. Sometimes, to improve the whiteness index (which measures the white cement) is added a little titanium oxide, on the basis of which it gets white pigments cement. This happens if the "white" is unsatisfactory.

Much more complicated than in the case of gray cement the normal manufacturing process makes the technical specification shall also include amounts of the various compounds that should be reduced to some specific and desired size. Necessity of precision makes the retail price or wholesale of such cement is much higher than "normal".

The data sheet is usually referred to:

• C₃S - that is alite - ingredient clinker. Responsible for initial strength and increases stamina. The general rule is that the more of it in the clinker, the faster it grows the initial strength
  
• C₂S - the belite - ingredient clinker, corresponding to the strength of the cement for a period longer than seven days, and the more of it, the faster the cement sets after the first few days
  
• C₃A - tricalcium aluminate - the high-frequency heat bonding, especially secreted in the early days, and in appropriate amounts in combination with alite and belite claims cement early strength. The addition of water accelerates even these processes.
  
• C₄AF - tetracalcium aluminoferrite - hardly affects the strength of the cement in the plus side, but in the manufacturing process serves as a flux. It is also responsible for undesirable color effects, making the cement is gray or shades of gray.
  

Research are determined

• MgO - magnesium oxide. Retarding influences on strength development. The amount thereof in the standards is limited to 6%.
  
• SO₃ - sulfuric anhydride. It is an indirect measure of the content of gypsum or anhydrite (calcium sulfate CaSO₄). Too much scale can dangerously accelerate them, thus usually the amount of SO₃ is subject to restrictions. If the amount of C₃A is more than 8%, the amount thereof must not exceed 3.5%, and if C₃A of less than 8% - SO₃ content may be at most 3%. European standards say about 4%. Its too much impact on the speed of reactions between the water and C₃A.
  
• Loss on ignition - they say if reduce the amount of cement in the sample after heating at 1832F (approx. 1000C). The bigger the loss, the lower the strength of cement. Standards are limited from 3 to 5%.
  
• The insoluble residue - say what part of the cement is insoluble in hydrochloric acid. Due to the fact that initially all the cement components are soluble, and the reaction with the calcium compounds become soluble, it shows the effectiveness and efficiency of the process. Standards establish a maximum limit of 0.75%, in Europe - 5%
  
• Alkali - alkali content, mainly chlorides, is reflected in the amount of potassium oxide K₂O and sodium oxide Na₂O. Large amounts can interfere with the processes of the cement, hinder the use of appropriate chemical modification of the setting time. Standards can be given by individual compounds or globally
  

Optional:

• False set - determines whether the cement has a disorder in the initial setting time, this is not a problem if the concrete is transported to the place of use or when it is pumped. Standard ATSM talking about the min. 50%.
  
• Autoclave expansion - is, in a sense bonding index delays caused by hydration of CaO or MgO or both of these oxides. Unfortunately, in many cases it is impossible to determine this accurately, thus sometimes performed another test, which measures the level of stability of cement. Rods are subjected to a temperature of 420F (approx. 215C) and a pressure of 295 psi (slightly more than 19 atm). Length change is limited to 0.8% for all types of cement.
  
• The air content - cements when mixed with sand and water tend to store air in itself. Because of this, it has an impact on many processes - its share is limited by the ATSM to 12%.